Formation of Nb₃Sn Using Mechanically Alloyed Nb – Sn Powder

S.N. Patankar  and F.H. (Sam) Froes

Institute for Materials and Advanced Processes, University of Idaho

Moscow ID  83844-3026

1.  Introduction

Superconductivity is generally regarded as a “macroscopic quantum phenomenon” wherein an element, inter-metallic alloy or compound conducts electricity without resistance. Type I superconductors are comprised of pure metals, whereas Type II superconductors are comprised primarily of alloys or intermetallic compounds. Nb₃Sn with A-15 type structure has high critical current density, j_c at very high magnetic fields is much sought after Type II superconductor. Because of their brittle nature, all the routes involving manufacturing of A-15 type superconductors make use of ductile precursors that are heat treated to form the A-15 compound by solid-state solution. In case of Nb₃Sn superconductor, the Nb₃Sn builds up at the Sn(bronze)/Nb interface and gradually spreads inside the Nb. The rate of progression is slow and reaction requires several hundreds of hours to reach completion. Use of mechanical alloyed (MA’d) Nb-Sn powder mixture could potentially ensure complete and near instantaneous transformation of Nb – Sn powder mixture to Nb₃Sn thereby, eliminating the need for independent long duration heat treatments that are normally required during the processing of Nb₃Sn superconductors. In our present study dealing with processing of Nb₃Sn, MA Nb – Sn was found to transform into Nb₃Sn during the heat treatment that followed. The objective of this paper is to discuss the formation of Nb₃Sn during the heat treatment of MA Nb – Sn powder mixture. 

2. Experimental details

The materials used in the present study were 99.9 % pure niobium powder, 3 – 7 micron in diameter and 99.9% pure tin, with sieve size of –325 mesh (<44 mm). High-energy milling a Model 8000 Spex mill was employed for MA. Milling was carried out “dry” in a carbide-lined steel cylinder under an Ar atmosphere to minimize oxidation. The charge to ball ratio was 1:10, the charge comprising of 10 g of a stoichiometric powder mixture. Structural transformation was monitored using the Philips x-ray diffractometer (XRD) with Cu K_a radiation. Perkin Elmer differential thermal analyzer (DTA-7) was used to study the phase transformation that occur during exposure of as-blended as well as milled powder to high temperature.

3. Results and Discussion

3.1 MA of Nb – Sn Powder Mixture.

Nb₃Sn did not form during the MA of Nb – Sn powder mixture. As shown in the XRD scan in figure 1, X-ray evidence for Nb₃Sn formation was lacking in powders milled for up to 3 h. Elemental niobium was present in significant quantities in the powder milled for 1 h, but the tin peaks were broadened so extensively by milling that the peak intensities were attenuated by a factor of ~10 or more. Extensive peak broadening for the Nb peaks was also observed in case of powder mixture milled for 3 h, figure 1. Increasing the milling time causes the peaks to broaden and decrease their intensities due to both the refinement of crystal size and introduction of strain during milling. When X-ray diffraction were obtained using a slow scan rate, it was observed that with increasing milling time the Nb peaks shifted to higher angles suggesting alloying of Sn with Nb leading to the formation of a solid solution. Nb has a BCC structure and is more brittle than Sn. During milling, the brittle Nb gets fragmented more easily than Sn, and gets coated on to the surface of the ductile Sn. Since X-ray diffraction occurs from a finite depth from the surface it is therefore likely that we observe the diffracted intensity from the BCC Nb phase and not from the Sn phase.

Figure 1: XRD Pattern of As-Blended and MA’d Nb – Sn Powder Mixture.

3.2  Heat Treatment of MA’d  Nb – Sn Powder Mixture

The MA’d Nb – Sn powder mixture was subsequently heat treated to obtain Nb₃Sn. The temperature and the heating rate to be maintained during the heat treatment was decided based on the thermal response of the powder mixture studied using differential thermal analyzer(DTA). Figure 2 is the DTA curve of the Nb – Sn powder mixture MA’d for one hour. The exothermic peak observed in the DTA curve probably corresponds to the temperature required to transform Nb – Sn powder mixture MA’d for 30 minutes to Nb₃Sn. This was confirmed by performing the X-ray diffraction study of the powder sample subjected to differential thermal analysis, figure 3.

Figure 2: DTA Curve of the Nb – Sn Powder Mixture Milled for One Hour.

Figure 3 shows the XRD Pattern of the MA’d Nb – Sn powder mixture subjected to DTA using a heating rate of 25 ^oC/Min.. From these XRD patterns it is apparent that even one hour of MA brings about complete transformation of Nb – Sn powder mixture to Nb₃Sn during the heat treatment that follows. Similar observations were made in case of Nb-Sn powder mixture MA’d for 30 minutes. Thus Nb – Sn powder mixture MA’d for short duration could be used as effective precursor for making Nb₃Sn superconductor using well-known powder in tube process or bronze process.

Figure 3: XRD Pattern of the MA’d Nb – Sn powder mixture subjected to DTA using a heating rate of 25 ^oC/Min.

3.3  Kinetic of Conversion of MA’D Nb – Sn Powder Mixture To Nb₃Sn

3.3.1    Kissinger method

Kissinger method has been used in the literature to determine the activation energy of solid state reaction that leads to the transformation of MA’D Nb – Sn powder mixture to Nb₃Sn.

As per the Kissinger equation

where A is the frequency factor and b is the heating rate, which is expressed as b = dT/dt.

Taking the logarithm of the above equation, we obtain

Activation energy, E_a can be obtained by plotting ln(bT_p²) as function 1/T_p, where T_p is the peak temperature.

2.2 Flynn-Wall method
This is one of the integral methods that can determine the activation energy which does not require the knowledge of reaction order.

(12)
where b the heating rate, A is the frequency factor, E_a is the activation energy and T  is the peak temperature. g(a) is integral function of conversion, a.
   

The activation energy for different conversion values can be calculated from a log b versus 1000/T plot.

Figure 4 : Isoconversional curves for MA’d Nb- Sn Powder from Kissinger and Flynn-Wall Expression

TABLE I : Kinetic Parameters for Conversion of MA’d Nb- Sn Powder to Nb₃Sn

The presently obtained activation energy for Nb₃Sn formation (312 kJ/mole) is 44 kJ/mole greater than that previously reported by Neijmeijer and Koster et al. Even though both the processes are diffusion controlled, the variation in the activation energy is due to the difference in nature of the reactants (precursors). In the present work Nb₃Sn was formed using elemental niobium and tin powder as the starting material, while Neijmeijer and Koster et al made Nb₃Sn via reaction between Nb₆Sn₅ powder and elemental niobium.

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